1. Field of the Invention
The present invention relates to a fuel injection control device, for an internal combustion engine, which executes a pilot fuel injection prior to a main fuel injection.
2. Description of the Related Art
Regarding a diesel engine, a pilot fuel injection, which injects a small amount of fuel into a combustion chamber prior to a main fuel injection, is known in the art.
In an internal combustion engine such as a diesel engine, diesel knock sometimes occurs. Diesel knock is a phenomenon which causes the combustion noise to increase. When the rate of pressure rise in the cylinder of the engine becomes excessively large due to, for example, a delay in ignition of the fuel, resonance in the combustion gas occurs. This causes a sudden increase in combustion noise, i.e., diesel knock.
It is generally known that the increase in combustion noise occurs under conditions where the ignition of fuel is delayed, such as in a cold start of the engine. Further, in a transient operating condition of the engine, the increase in combustion noise may occur due to delay in the change in the temperature of intake air and combustion chamber of the engine. Further, in an engine employing a high pressure fuel injection, as combustion speed increases due to high fuel injection pressure, an increase in combustion noise may occur.
It is known in the art that a pilot fuel injection, which injects a small amount of fuel in the cylinder prior to the main fuel injection, is effective for preventing the increase in combustion noise. As fuel injected in into the combustion chamber by the pilot fuel injection burns before the main fuel injection is executed, the pressure and temperature in the combustion chamber increase to suitable conditions for combustion of fuel and, thereby, the delay in the ignition of the fuel injected by the main fuel injection is shortened and an increase in combustion noise is suppressed.
An example of the internal combustion engine employing a pilot fuel injection is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-18074. In the ""074 publication, a plurality of pilot fuel injections, each injecting a small amount of fuel, are performed in order to prevent liquid fuel drops from reaching the cylinder wall. When liquid fuel injected by a fuel injection reaches the cylinder wall without evaporating, the liquid fuel dilutes lubrication oil on the cylinder wall and causes insufficient lubrication. In the ""074 publication, the amount of the fuel injected by each pilot fuel injection is reduced so that all the fuel inject by each pilot fuel injection evaporates before reaching the cylinder wall.
In a diesel engine employing a high pressure fuel injection system, which is prone to a diesel knock, a pilot fuel injection is employed in order to suppress the diesel knock.
In the high pressure fuel injection system, usually, a pressure balance type fuel injection valve is used in which the valve element is pressed against the valve seat by the pressure of the fuel in the fuel injection valve housing. The valve opening pressure at which the fuel injection valve opens is determined by an equilibrium between fuel pressure exerted on the valve element on the side where it contacts the valve seat and the fuel pressure exerted on the valve element on the side opposite thereto in the pressure-balance type fuel injection valve.
During the operation of the pressure-balance type fuel injection valve, the valve element and the valve seat collide each other in the fuel injection operation, whereby the valve seat gradually wears.
When the valve seat wears, the aperture of the valve seat, where the tip of the valve element fits, is enlarged. This causes a decrease in the area of the portion of the valve element located outside of the aperture. As the fuel pressure exerted on the portion of the valve element outside of the aperture acts in the direction for opening the fuel injection valve, when the area of this portion of the valve element decreases, a higher fuel pressure is required in order to lift the valve element from the valve seat. In other words, the valve opening pressure increases as the wear of the valve seat increases.
When the valve opening pressure becomes higher, the time required for the fuel pressure in the fuel injection valve to increase to the valve opening pressure after the fuel injection command signal is transmitted becomes longer even if the length of the fuel injection command signal is the same. As the length of the fuel injection command signal corresponds to a target amount of fuel injection, when the time required for the fuel pressure to increase the valve opening pressure increases, the actual time period where the valve element is away from the valve seat (the actual fuel injection period) becomes shorter and the actual fuel injection amount becomes smaller even if the length of the fuel injection command signal becomes smaller.
Thus, the fuel injection characteristics, such as the relationship between the length of the fuel injection command signal and the actual fuel injection amount, or a valve opening pressure, change as the operation period of the fuel injection valve becomes longer. In this case, even if the length of the fuel injection command signal is constant, the actual fuel injection amount gradually decreases.
The magnitude of decrease in the fuel injection amount due to the wear of the valve seat is relatively small. However, as the amount of fuel injected by a pilot fuel injection is small, this decrease in the fuel injection amount adversely affects the pilot fuel injection in some cases.
Therefore, in the diesel engine employing pilot fuel injection, the target amount of the pilot fuel injection is usually set at a value larger than the amount actually required so that a sufficient amount of the pilot fuel injection can be obtained even after wear of the valve seat, due to a long operating period, occurs.
However, when the target amount of the pilot fuel injection is set at a value larger than the amount actually required for the pilot fuel injection, the actual amount of the pilot fuel injection becomes excessively large in some cases. If an excess amount of fuel is injected in the pilot fuel injection, the dilution of lubricating oil by liquid fuel drops reaching the cylinder wall, or exhaust smoke due to excess amount of pilot fuel injection, occur.
On the other hand, if the target amount of the pilot fuel injection is set at a value which does not include the margin, the amount of the pilot fuel injection becomes insufficient after a long operating period and, thereby, the diesel knock or deterioration of exhaust emission occurs in some cases.
In view of the problems in the related art as set forth above, the object of the present invention is to provide a fuel injection control device capable of controlling the fuel injection amount to exactly required value by detecting changes in fuel injection characteristics of the fuel injection valve and correcting the fuel injection command signal in accordance with the changes in the fuel injection characteristics.
The object as set forth above is achieved by a fuel injection control device, according to the present invention, comprising a reservoir storing high pressure fuel, a fuel injection valve connected to the reservoir and injects fuel stored in the reservoir into a combustion chamber of an internal combustion engine, the device executing a pilot fuel injection by injecting fuel into the combustion chamber prior to a main fuel injection, wherein the device further comprises fluctuation detecting means for detecting a magnitude of the fluctuation of pressure caused by the pilot fuel injection in fuel supplied to the fuel injection valve, correction means for calculating a magnitude of change in pilot fuel injection characteristics of the fuel injection valve from a standard fuel injection characteristics based on the magnitude of the pressure fluctuation detected by the fluctuation detecting means and for correcting the amount of the pilot fuel injection based on the change in the pilot fuel injection characteristics in such a manner that the actual amount of fuel injected by the pilot fuel injection agrees with a target pilot fuel injection amount.
According to the present invention, in a fuel injection system equipped with a reservoir (a common rail), the change in the fuel injection characteristics of the fuel injection valve from the standard fuel injection characteristics is calculated based on the magnitude of the fluctuation of fuel pressure accompanying the pilot fuel injection.
When the pilot fuel injection is executed, the fuel pressure in the fuel system fluctuates and the magnitude of the fluctuation changes in accordance with the pilot fuel injection amount. For example, when the pilot fuel injection amount decreases due to wear of the valve seat, the magnitude of the fluctuation of fuel pressure accompanying the pilot fuel injection decreases accordingly.
Therefore, it is possible to determine the magnitude of change in fuel injection characteristics, such as the fuel injection amount, from the fuel pressure fluctuation accompanying the pilot fuel injection. Further, it is also possible to correct a fuel command signal in accordance with the change in the fuel injection characteristics so that the actual pilot fuel injection amount agrees with a target amount. Therefore, the actual pilot fuel injection amount can be adjusted to the target value regardless of the state of the fuel injection valve, for example, from the state when the fuel injection valve is new to the state the fuel injection valve has been used for long time. Thus, the pilot fuel injection amount is always controlled to suitable value and, thereby, dilution of the lubricating oil or deterioration of the exhaust gas property does not occur.
Further, the change in the fuel injection characteristics affects the main fuel injection amount in the same manner as that on the pilot fuel injection amount. Therefore, the main fuel injection amount can also be corrected based on the change in the fuel injection characteristics in pilot fuel injection. Thus, it is also possible to correct the main fuel injection amount based on the change in the fuel injection characteristics in pilot fuel injection.
Further, as the fuel injection characteristics change in accordance with, for example, wear of the valve seat of the fuel injection valve, it is possible to determine the degree of deterioration of the fuel injection valve (such as wear of the valve seat) based on the magnitude of change in the fuel injection characteristics. For example, if the change in the fuel injection characteristics is large, it can be determined that wear of the valve has increased to an impermissible degree. Further, when the pilot fuel injection amount or the main fuel injection amount is corrected in accordance with the change in the fuel injection characteristics in such a manner that the actual fuel injection amount agrees with the target value, the correction amount of the fuel injection amount corresponds to the change in the fuel injection characteristics. Therefore, degree of deterioration of the fuel injection valve can be determined based on the correction amount of the pilot fuel injection amount or the main fuel injection amount.